A common mode rejection circuit serves the purpose to reject a common mode and a resulting common mode current through an electrical circuit, for example, through an electrical circuit of a radio frequency transceiver of a mobile communication device or through an oscillator. Such a common mode is an interference caused by the oscillator or by the electrical circuit that operates at a frequency. For example, the oscillator that comprises a differential amplifier driven with a high signal swing suffers from large signal effects like the common mode. The common mode appears on the signal outputs of the oscillator as well as on ground and the power supply. This common mode signal or the main oscillation signal may lead to creation of a voltage swing between ground and power supply or even beyond of a DC supply at twice the input frequency oscillator.
Without a common mode rejection or with limited common mode rejection this voltage swing leads to the common mode current through the oscillator. The common mode current may negatively effect signal purity at high oscillation frequencies. Furthermore, with the appearance of common mode signals an additional noise conversion typically up-converts flicker noise to oscillation frequencies. The common mode has a further impact on the current consumption. This is clear if one considers the quality factor (Q factor) of an oscillator or resonator defined by the ratio of the stored energy to the dissipated energy. The common mode currents of the second harmonic frequency do not contribute to the stored energy, but to the dissipated energy. Therefore, the common mode current lowers the quality factor of the oscillator and thus may increase the losses of the oscillator without a common mode rejection circuit or with a limited common mode rejection circuit. Finally this may cause increased power consumption for a certain phase noise performance.